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Climate Data Hint at Irreversible Rise in Seas

by: ANDREW C. REVKIN    24 March 2006

Within the next 100 years, the growing human influence on Earth's climate could lead to a long and irreversible rise in sea levels by eroding the planet's vast polar ice sheets, according to new observations and analysis by several teams of scientists.

One team, using computer models of climate and ice, found that by about 2100, average temperatures could be four degrees higher than today and that over the coming centuries, the oceans could rise 13 to 20 feet — conditions last seen 129,000 years ago, between the last two ice ages.

The findings, being reported today in the journal Science, are consistent with other recent studies of melting and erosion at the poles. Many experts say there are still uncertainties about timing, extent and causes.

But Jonathan T. Overpeck of the University of Arizona, a lead author of one of the studies, said the new findings made a strong case for the danger of failing to curb emissions of carbon dioxide and other gases that trap heat in a greenhouselike effect.

"If we don't like the idea of flooding out New Orleans, major portions of South Florida, and many other valued parts of the coastal U.S.," Dr. Overpeck said, "we will have to commit soon to a major effort to stop most emissions of carbon to the atmosphere."

According to the computer simulations, the global nature of the warming from greenhouse gases, which diffuse around the atmosphere, could amplify the melting around Antarctica beyond that of the last warm period, which was driven mainly by extra sunlight reaching the Northern Hemisphere.

The researchers also said that stains from dark soot drifting from power plants and vehicles could hasten melting in the Arctic by increasing the amount of solar energy absorbed by ice.

The rise in sea levels, driven by loss of ice from Greenland and West Antarctica, would occur over many centuries and be largely irreversible, but could be delayed by curbing emissions of the greenhouse gases, said Dr. Overpeck and his fellow lead author, Bette L. Otto-Bliesner of the National Center for Atmospheric Research in Boulder, Colo.

In a second article in Science, researchers say they have detected a rising frequency of earthquakelike rumblings in the bedrock beneath Greenland's two-mile-thick ice cap in late summer since 1993. They say there is no obvious explanation other than abrupt movements of the overlying ice caused by surface melting.

The jostling of that giant ice-cloaked island is five times more frequent in summer than in winter, and has greatly intensified since 2002, the researchers found. The data mesh with recent satellite readings showing that the ice can lurch toward the sea during the melting season.

The analysis was led by Goran Ekstrom of Harvard and Meredith Nettles of the Lamont-Doherty Earth Observatory in Palisades, N.Y., part of Columbia University.

H. Jay Zwally, a NASA scientist studying the polar ice sheets with satellites, said the seismic signals from ice movement were consistent with his discovery in 2002 that summer melting on the surface of Greenland's ice sheets could almost immediately spur them to shift measurably. The meltwater apparently trickles through fissures and lubricates the interface between ice and underlying rock.

"Models are important, but measurements tell the real story," Dr. Zwally said. "During the last 10 years, we have seen only about 10 percent of the greenhouse warming expected during the next 100 years, but already the polar ice sheets are responding in ways we didn't even know about only a few years ago."

In both Antarctica and Greenland, it appears that warming waters are also at work, melting the protruding tongues of ice where glaciers flow into the sea or intruding beneath ice sheets, like those in western Antarctica, that lie mostly below sea level. Both processes can cause the ice to flow more readily, scientists say.

Many experts on climate and the poles, citing evidence from past natural warm periods, agreed with the general notion that a world much warmer than today's, regardless of the cause of warming, will have higher sea levels.

But significant disagreements remain over whether recent changes in sea level and ice conditions cited in the new studies could be attributed to rising concentrations of the greenhouse gases and temperatures linked by most experts to human activities.

Sea levels have been rising for thousands of years as an aftereffect of the warming and polar melting that followed the last ice age, which ended about 10,000 years ago. Discriminating between that residual effect and any new influence from human actions remains impossible for the moment, many experts say.

Satellites and tide gauges show that seas rose about eight inches over the last century and the pace has picked up markedly since the 1990's.

Dr. Overpeck, the co-author of the paper on rising sea levels, acknowledged the uncertainties about the causes. But he said that in a world in which humans, rich and poor, increasingly clustered on coasts, the risks were great enough to justify prompt action.

"People driving big old S.U.V.'s to their favorite beach or coastal golf course," he said, should "start to think twice about what they might be doing."